Recently, many researches are being conducted on Multiple Input Multiple Out (MIMO) system using spatial resource for transmitting high quality data at high data rate in a wireless communication environment.
The MIMO system can be classified into one of Single-User MIMO using the given time-frequency resource for one user entirely and a Multi-User MIMO (MU-MIMO) using the given time-frequency resource for multiple users in common through spatial multiplexing.
In the current 3rd Generation Project Partnership Long Term Evolution (3GPP LTE) downlink communication mode, the base station transmits the data precoded with a unitary matrix based on Precoding Matrix Index (PMI), Channel Quality Indicator (CQI), and Rank Indicator (RI) reported by terminals.
In the 3GPP LTE downlink communication mode, the terminal determines the PMI, CQI, and RI based on the current channel state and received signal quality level and feeds back the PMI, CQI, and RI.
In the conventional SU-MIMO system, the researches has been focused on the case where the number of receive antennas is identical with the number of transmission streams. In the SU-MIMO, a best match candidate selected in the codebook, particularly Grassmanian codebook, is transmitted to the transmitter to decrease transmission error rate and increase transmission throughput. The Grassmaniann codebook makes it possible to achieve the highest performance in SU-MIMO logically.
In the case of MU-MIMO, if the transmitter does not know the accurate channel information, it is difficult to achieve maximum multiplexing gain due to the inter-user interference. Since it is difficult for the base station to acquire accurate channel information, the codebook-based MU-MIMO technology is still used. The Grassmanian codebook or orthogonal codebook is of being used usually.
The orthogonal codebook is characterized in that the candidate vectors are not correlated as far as possible as thus shows good performance as the number of candidate users increases even with its design without reflecting channel characteristic
The Grassmanian codebook is characterized in that the channel is quantized by in the way of feedback of the candidate vector closest to the channel direction in itself. In the case that the number of candidate users is not so many, the Grassmanian codebook may show the better performance as compared to the orthogonal codebook but also has a drawback in that it is difficult to form a codebook of a certain size.
The SU-MIMO or MU-MIMO allocates the time-frequency resource to one user or multiple users within a cell. Also, the studies on the resource allocation for multi-cell environment have been conducted separately. The multi-cell MIMO is being discussed in 3GPP along with joint processing technology, coordinated beamforming technology, and the like. These technologies are being studied in consideration of transmitting user-specific messages.
However, the multi-cell technology is designed mainly based on the Grassmanian codebook and orthogonal codebook still with the focus on the case where the number of receive antennas are equal to the number of streams. In the case that the number of receive antennas is greater than the number of streams, this requires optimization of the transceiver as such and iteration in the normal situation and risks divergence at times.